19.03.txt

Exercise 19.3: Given the following class hierarchy in which each class defines
a public default constructor and virtual destructor:

	class A { /* . . .  */ };
	class B : public A { /* . . .  */ };
	class C : public B { /* . . .  */ };
	class D : public B, public A { /* . . .  */ };

which, if any, of the following dynamic_casts fail?

	(a) A *pa = new C;
		B *pb = dynamic_cast< B* >(pa);
	(b) B *pb = new B;
		C *pc = dynamic_cast< C* >(pb);
	(c) A *pa = new D;
		B *pb = dynamic_cast< B* >(pa);

By Faisal Saadatmand

Refer to section 19.2.1, p. 825 for rules governing the dynamic_cast operator.

(a) succeeds; target type, B, is a public base class of the type denoted by pa, C.
(b) fails; C is not a public base type of B, and the dynamic type denoted by
	pb, B is not the same as target type, C, or contains subobjects the target
	type.
(c) A *pa = new D cannot be evaluated because of the ambiguity in the multiple
	inheritance paths to A. To fix the ambiguity, B and D must inherit
	virtually from A. In that case, the cast will succeeds because B is a
	public base class of D.  Note: I am not quite sure about this, but I
	believe that since A is virtually inherited, i.e. there is only one
	instance of A, the (down) cast will not result in loss of data.